One commonly used method for producing polymers is gas phase polymerization. During operation to produce polyolefins by polymerization, a conventional gas phase fluidized bed reactor contains a fluidized dense-phase bed including a mixture of reaction gas, polymer (resin) particles, catalyst, and catalyst modifiers. Before such a polymerization reaction, a “seed bed” is typically loaded into the reactor or is present in the reactor from a previous polymerization operation. The seed bed is (or consists essentially of) granular material that is or includes polymer material. The polymer material can but need not be identical to the desired end product of the reaction. An example of seed bed material is metallocene polyethylene.
It is known to introduce a continuity additive (“CA”) into a reactor during a fluidized bed polymerization reaction to reduce sheeting and/or fouling in the reactor during polymerization. Such use of a continuity additive, optionally with a flow improver, is described in U.S. Pat. No. 6,482,903, issued Nov. 19, 2002; U.S. Pat. No. 6,660,815, issued Dec. 9, 2003; U.S. Pat. No. 6,306,984, issued Oct. 23, 2001; and U.S. Pat. No. 6,300,436, issued Oct. 9, 2001, all assigned to the assignee of the present invention. A continuity additive is typically not catalytic, but is typically combined with a catalyst (and typically also with a flow improver) before or after being introduced into the reactor. Examples of CAs are aluminum stearate, other metal stearates, and Atmer AS 990 (an ethoxylated stearyl amine, available from Ciba Specialty Chemicals Co, Basel, Switzerland).
U.S. Pat. No. 6,300,436 and U.S. Pat. No. 6,306,984 describe an olefin polymerization process (e.g., a gas phase or slurry phase process) in a reactor the presence of a catalyst composition comprising a carboxylate metal salt. The carboxylate metal salt is a continuity additive (“CA”) which significantly reduces sheeting and/or fouling in the reactor during polymerization. The catalyst composition is produced by combining, contacting, blending and/or mixing a catalyst system (e.g., a supported catalyst system) with the carboxylate metal salt. The catalyst system can be a transition metal catalyst compound (e.g., a bulky ligand metallocene-type catalyst compound). The carboxylate metal salt can be blended (e.g., tumble dry blended) with a supported catalyst system or polymerization catalyst comprising a carrier. The polymerization catalyst can be dry and free flowing and the metal carboxylate salt mixed or blended with the catalyst can be in solid form. Alternatively, the carboxylate metal salt is added to a reactor (containing reactants and a catalyst system) during polymerization without previously having been combined, blended, contacted, or mixed with the catalyst system.
U.S. Pat. No. 6,300,436, U.S. Pat. No. 6,306,984, and U.S. Pat. No. 6,482,903 teach that carboxylate metal salts that may be suitable for use as continuity additives are any mono- or di- or tri-carboxylic acid salt with a metal portion from the Periodic Table of Elements. Examples include saturated, unsaturated, aliphatic, aromatic or saturated cyclic carboxylic acid salts where the carboxylate ligand has preferably from 2 to 24 carbon atoms, such as acetate, propionate, butyrate, valerate, pivalate, caproate, isobuytlacetate, t-butyl-acetate, caprylate, heptanate, pelargonate, undecanoate, oleate, octoate, palmitate, myristate, margarate, stearate, arachate and tercosanoate. Examples of the metal portion includes a metal from the Periodic Table of Elements selected from the group of Al, Mg, Ca, Sr, Sn, Ti, V, Ba, Zn, Cd, Hg, Mn, Fe, Co, Ni, Pd, Li and Na.
Examples of carboxylate metal salts that may be suitable for use as continuity additives are represented by the general formula M(Q)x(OOCR)y, where M is a metal from Groups 1 to 16 and the Lanthanide and Actinide series, preferably from Groups 1 to 7 and 13 to 16 (preferably Groups 2 and 13, and most preferably Group 13); Q is a halogen or hydrogen, or a hydroxy, hydroxide, alkyl, alkoxy, aryloxy, siloxy, silane sulfonate group, or siloxane; R is a hydrocarbyl radical having from 2 to 100 carbon atoms, preferably 4 to 50 carbon atoms; and x is an integer from 0 to 3 and y is an integer from 1 to 4 and the sum of x and y is equal to the valence of the metal. In a preferred embodiment of the above formula, y is an integer from 1 to 3, preferably 1 to 2, especially where M is a Group-13 metal.
Non-limiting examples of R in the above formula include hydrocarbyl radicals having 2 to 100 carbon atoms that include alkyl, aryl, aromatic, aliphatic, cyclic, saturated or unsaturated hydrocarbyl radicals. For example, R can be a hydrocarbyl radical having greater than or equal to 8 carbon atoms (preferably greater than or equal to 17 carbon atoms) or R can be a hydrocarbyl radical having from 17 to 90 carbon atoms (preferably from 17 to 54 carbon atoms).
Non-limiting examples of Q in the above formula include one or more, same or different, hydrocarbon containing group such as alkyl; cycloalkyl, aryl, alkenyl, arylalkyl, arylalkenyl or alkylaryl, alkylsilane, arylsilane, alkylamine, arylamine, alkyl phosphide, alkoxy having from 1 to 30 carbon atoms. The hydrocarbon containing group may be linear, branched, or even substituted. For example, Q can be an inorganic group such as a halide, sulfate or phosphate.
For some applications, a carboxylate metal salt employed as a CA has a melting point from about 30° C. to about 250° C. (preferably from about 100° C. to about 200° C.). For some applications, the carboxylate metal salt employed as a CA is an aluminum stearate having a melting point in the range of from about 135° C. to about 65° C. For typical applications, the carboxylate metal salt employed as a CA has a melting point greater than the polymerization temperature in the reactor.
Other examples of carboxylate metal salts that may be suitable for use as continuity additives include titanium stearates, tin stearates, calcium stearates, zinc stearates, boron stearates and strontium stearates.
For some applications, a carboxylate metal salt is combined (for use as a continuity additive) with an antistatic agent such as a fatty amine, for example, Atmer AS 990/2 zinc additive, a blend of ethoxylated stearyl amine and zinc stearate, or Atmer AS 990/3, a blend of ethoxylated stearyl amine, zinc stearate and octadecyl-3,5-di-tert-butyl-4-hydroxyhydrocinnamate. Both the AS 990/2 and 990/3 blends are available from Crompton Corporation of Memphis, Tenn.
U.S. Pat. Nos. 6,482,903 and 6,660,815 teach performance of an olefin polymerization process (e.g., a gas phase or slurry phase process) in a reactor in the presence of a catalyst composition including a catalyst system (e.g., a supported bulky ligand metallocene-type catalyst system), at least one carboxylate metal salt, and at least one flow improver. The flow improver can be a colloidal particulate material (e.g., Snowtex colloidal silica, available from Nissan Chemical Industries, Tokyo, Japan, or another colloidal silica). Other examples of the flow improver that are disclosed in U.S. Pat. No. 6,482,903 include a colloidal silica (e.g., Cabosil, available from Cabot), a fumed silica, a syloid, and alumina. U.S. Pat. Nos. 6,482,903 and 6,660,815 teach that the carboxylate metal salt is preferably contacted with the flow improver prior to use in the reactor or contact with a polymerization catalyst, and that a catalyst system can be combined, contacted, blended, or mixed with a composition of at least one carboxylate metal salt and at least one flow improver before use in a reactor.
U.S. Pat. Nos. 6,482,903 and 6,660,815 also teach that because carboxylate metal salts are difficult to handle (e.g., because their morphology is poor and because they have low bulk density and fluffy consistency), a combination of a carboxylate metal salt and a flow improver can be handled and combined with a supported catalyst system in a substantially improved manner than can the carboxylate metal salt alone.
U.S. Pat. Nos. 6,300,436 and 6,306,984 teach that when starting up a polymerization reaction, especially a gas phase process, there is a higher tendency for operability problems to occur. They also teach performing the initial stages of such a reaction (before the process has stabilized) in the presence of a polymerization catalyst and carboxylate metal salt mixture to reduce or eliminate start-up problems. They also teach implementing a transition after the initial stages of the reaction (i.e., when the reactor has begun to operate in a stable state) to cause the reaction to proceed in the presence of the same (or a different) polymerization catalyst but not in the presence of the carboxylate metal salt.
However, the present inventors have recognized that a reactor can be vulnerable to sheeting and/or fouling during the critical initial stage(s) of a polymerization reaction (before the reaction has stabilized) even if each such initial stage is performed in the presence of a CA, if the concentration of the CA is low. The present inventors have also recognized that the concentration of CA in a reactor is typically too low to eliminate this vulnerability if the CA is introduced during the initial stage(s) of the polymerization reaction (i.e., after the reaction has begun).
Before the present invention, it had not been known how reliably to prevent sheeting and/or fouling during the critical initial stage(s) of a polymerization reaction.